There seem to be an almost endless supply of distractions when we are writing, and even your computer can be one.
Ideally your computer desktop should be almost clear, but if you're like me that's not the case. There are all kinds of files on there, as well as the dock that alerts me with a little red symbol when a new email has come in, also easy access to iTunes with its endless supply of podcasts, and on and on.
One way to reduce distractions is to go full screen when you are writing. Because I have a larger screen, simply clicking 'full screen' from the Word menu doesn't do it. But there are word processors that strip the process down (not good for scriptwriting, but fine for prose).
If you're a Mac person, try "WriteRoom" from hogbaysoftware.com -- you can get a free 30-day trial and if you like it, it will cost you $25. It creates a blank screen with green letters or you can choose your own colors, fonts, and page layout. If you suffer from eyestrain, you can scale up your test for easier reading. It offers spell-checking as well.
If you're on Windows, try out DarkRoom. If you have alternatives that you like for either Mac or Windows, please add them in the comments here.
Or, of course, you could just clear that desktop...